1. Field of the Invention
This invention involves a process for the utilization of an unusual memory characteristic which is present in neutralized sulfonated elastomeric materials, wherein the process includes the steps of distorting or compressing the neutralized sulfonated elastomeric article, then heating the article to a selected elevated temperature, holding the article at the elevated temperature for a limited time while maintaining the distorted shape, cooling the article to near room temperature, and releasing it from the device used to compress or distort it. The released cooled elastomer does not return to its original form, but rather substantially retains the distorted shape for an indefinite period of time. The elastomer in the distorted shape is heated to an appropriate elevated temperature thereby allowing the article to recover towards its original undistorted shape.
2. Description of the Prior Art
The present unique and novel invention relates to a fabrication process for reforming a neutralized sulfonated elastomer by utilization of an unusual memory property of these elastomers. This fabrication process permits production of a metastable state of these elastomeric articles which on simple heating undergoes a hystersis to approximately their original shape. Thus, for example, by this process an elastomeric article can be compressed and maintained in a metastable distorted shape for compact storage or transport; and then subsequently, it can be heated to an elevated temperature to cause it to recover towards its original shape.
The memory characteristic with which this invention is concerned has been described previously for thermoplastic polymer systems, but not for elastomeric polymers due to their extremely low glass transitions. More specifically, this characteristic has typically been limited to polymers which possess a glass transition or crystalline melting point well above room temperature, and which also are chemically covalently crosslinked to provide the permanent restoring forces. For example, chemically crosslinked polyethylene is well known in the art. This polymer can be molded above the crystalline melting point of the polyethylene, and then chemically crosslinked in the molded form (usually by irradiation) to result in permanent covalent crosslinks. Thus, if subsequently heated even much above the crystalline melting point, the polymer will not flow; and, even if deformed, the polymer will attempt to return to its equilibrium state which is that molded shape in which it was chemically covalently crosslinked. If heated to above the crystalline melting point and stressed to a different configuration, and then cooled in this configuration to below the melting point of the polyethylene, the polymer will retain indefinitely the deformed configuration at ambient temperature. If, however, the polymer is heated again above the melting point of the polyethylene crystallinity, the polymer will respond to the previously locked-in stresses and achieve the configuration which it held during the chemical crosslinking procedure. As noted above, this behavior for plastics is based on two critical features: the polymer must possess a glass transition or crystalline melting point above room temperature, and it must be chemically covalently crosslinked to provide the permanent restoring forces. This latter requirement has the result that the plastic becomes thermoset. That is, it is no longer melt processable or melt reprocessable.
U.S. Pat. No. 4,053,548 describes a new process for exhibiting a memory effect in plastic materials which is based on selected multiphase plastic block copolymers, and also by plastics possessing ionic domains. These latter systems possess the added advantage of processability which is a very important attribute heretofore not readily available with such memory conferring systems.
However, these systems are also plastic in nature (i.e. a high stiffness modulus at room temperature in excess of about 10,000 psi). And, as with the other systems exhibiting the memory characteristic, these systems also depend on the existence of glass transitions of crystalline melting points above room temperature, and they are not rubbery at normal use temperatures as are the sulfonated elastomeric polymers of the present invention.
The sulfonated elastomeric compositions of the present invention are derived from elastomeric polymers having a hydrocarbon backbone with olefinic unsaturation, especially elastomeric polymers such as Butyl and EPDM rubbers. The process of sulfonating polymers is described in U.S. Pat. Nos. 3,642,728 and 3,836,511, herein incorporated by reference, and U.S. Pat. Nos. 3,870,841 and 3,847,854 herein incorporated by reference, which teach methods of plasticization of these sulfonated polymers. U.S. Pat. Nos. 3,867,319 and 3,947,387 by Lundberg, herein incorporated by reference, teach a process for the formation of an expanded foam of a sulfonated thermoplastic. However, these patents fail to describe or ascertain the ability of sulfonated elastomeric articles to exhibit the novel memory effect for elastomers with which this present invention is concerned. It is surprising that a sulfonated elastomeric article which has its glass transition substantially below room temperature can exhibit a memory effect similar to that of plastic materials. Accordingly, the present instant invention provides a unique and novel process for forming a metastable state of a sulfonated elastomer which contains locked-in stresses, and which is later recovered towards its original shape by heating.